This invention relates to a method for forming perforations in barshaped articles.
The invention finds particularly advantageous application in the production of smoking articles, in particular cigarettes, to which the description given hereinafter specifically refers but without thereby limiting its general application.
In cigarette manufacture, it is known to form so-called "ventilated" cigarettes, i.e. cigarettes provided in the filter zone with a plurality of perforations which enable the smoker to inhale a percentage of external air together with the smoke, with the double advantage of diluting the inhaled smoke and reducing its temperature and thus its harmful element content.
In forming said ventilation perforations it is known to use perforation devices comprising a laser source, possibly of pulsed type, and a focusing device arranged to direct the laser beam on to the cigarettes as they advance along a determined path and possibly rotate about their axis.
In particular, a perforation device is known in which the cigarettes to be perforated are transferred by an inlet roller to a perforation roller which is disposed tangential to said inlet roller and is provided along its outer periphery with a plurality of rotatable cradles each arranged to receive a respective cigarette to be perforated and, when perforated, to transfer it to an outlet roller tangential to said perforation roller.
As it passes through the positions in which it is tangential to the inlet and outlet rollers, each cradle assumes a determined angular rest position relative to the perforation roller in order to facilitate correct transfer of the cigarettes between the inlet, perforation and outlet rollers. While it traverses a central portion of its path of advancement between the two said points of tangency, each cradle is rotated firstly at increasing angular speed, then at constant angular speed, and then at decreasing angular speed. Each cigarette is perforated as it advances along a perforation arc, along which the relative cradle rotates about its axis at the said constant angular speed so as to successively expose determined points of the cigarette periphery to the action of said pulsed laser beam.
In the aforesaid known perforation device, the cigarettes are subjected to considerable positive and negative angular acceleration as they advance along the perforation roller, and this can compromise their structural stability, at least causing tobacco to escape. Moreover, as a result of the need to angularly accelerate and then decelerate said cradles, the length of the perforation arc is relatively small. For this reason, very powerful laser sources have to be used as the energy necessary for perforation must be transferred to each cigarette within an extremely short time.
Finally, from the mechanical aspect the aforesaid known perforation device has a relatively complicated structure due to the need to vary the angular speed with which the cradles rotate about their axis between zero and a determined constant value during each complete revolution of the perforation roller about its axis.